1. Field of the Invention
The present invention relates to a relief valve, in which a main poppet is provided with an inflow fluid passage and an annular fluid passage communicating a high-pressure inlet and a back pressure chamber, so that a flow rate of hydraulic fluid is not greatly changed even when temperature is changed, and thus providing smooth operation and preventing foreign materials contained in the hydraulic fluid from being introduced into the relief valve.
2. Description of the Related Art
In general, heavy equipment such as an excavator, a payloader, a skidder, etc., which is operated with a hydraulic pressure employs a hydraulic system in order to drive various working tools mounted thereon by use of hydraulic fluid supplied from a hydraulic pump. The hydraulic system of the heavy equipment is provided with a relief valve which functions to regulate the maximum pressure in order to protect the entire hydraulic system including a driving source from an overload.
The relief valve is one of pressure control valves, which discharge a part or total of fluid when a pressure of the hydraulic system reaches a setting pressure of the valve, thereby performing a function of keeping the pressure in the hydraulic system under a set value. The relief valve is capable of changing the setting pressure, so that it can cope with the case of changing the actuator of the equipment.
FIG. 1 illustrates a cross-sectional view of a conventional relief valve.
According to the conventional relief valve 100, hydraulic fluid is supplied from a hydraulic pump 200 to a high-pressure inlet 111 of the relief valve 100, so that the whole pressure of a hydraulic system is exerted on the high-pressure inlet 111. The relief valve discharges the hydraulic fluid from the high-pressure inlet 111 toward a tank 201 when the pressure of the high-pressure inlet 111 reaches a predetermined pressure or more, thereby protecting the hydraulic system from an overload.
The relief valve 100 is comprised of a sleeve 110, and a main body 180 coupled to the sleeve 110. The sleeve 110 is provided therein with a main poppet 120, a main poppet spring 122, a second orifice body 130 and a seat 140.
The main poppet 120 is provided with a first orifice 121 at a central portion thereof, and is fitted to be movable in a longitudinal direction in the sleeve 110. The main poppet spring 122 resiliently supports the main poppet 120.
Further, the main poppet spring 122 has a rear end supported by the second orifice body 130. The second orifice body 130 is installed so that its rear end is supported on the seat 140. The second orifice body 130 is provided with a second orifice 131 which passes through the central portion of the second orifice body 130.
The main body 180 coupled with the sleeve 110 is provided therein with a pilot poppet 150, a pilot poppet spring 160, and an adjustable piston 170. The pilot poppet 150 is resiliently supported by the pilot poppet spring 160, and is provided to be movable in a longitudinal direction. The pilot poppet spring 160 is supported by the adjustable piston 170 on a rear end thereof.
The pilot poppet 150 takes a conical shape, and is brought into contact with the seat surface 141 of the seat 140 by a spring force of the pilot poppet spring 160, thus functioning to open/close the seat fluid channel 142.
The adjustable piston 170 supporting the pilot poppet spring 160 is capable of moving relative to the main body 180 and is fixed to the main body 180 by a fixing nut 171. Thus, user can change the setting pressure of the relief valve 100, by moving the adjustable piston 120 left or right to adjust the tension of the pilot poppet spring 160, and then fastening the fixing nut 171 to fix the adjustable piston 170 to the sleeve 110.
There is provided a tank inlet 181 connected to a tank fluid channel 112 in the rear of the pilot poppet 150. When the pilot poppet 150 opens the seat 140, the hydraulic fluid, which has passed through a seat fluid channel 142, is discharged through the tank inlet 181 to the tank 201.
The following description will be made regarding the operation of the relief valve having the foregoing construction. Hereinafter, a pressure exerted on the side of the high-pressure inlet 111 by the hydraulic fluid from the hydraulic pump 200 is referred to as a “inlet side pressure”, and a pressure exerted on a space 120a of the main poppet 120 before the hydraulic fluid passing through the first orifice 121 of the main poppet 120 is introduced into the second orifice 131 is referred to as a “chamber side pressure”.
The hydraulic fluid from the hydraulic pump 200 is introduced into the space 120a of the main poppet 120 via the high-pressure inlet 111 and the first orifice 121. When the inlet side pressure is lower than the setting pressure set by the pilot poppet spring 160, the pilot poppet 150 is brought into contact with the seat surface 141 by the spring force of the pilot poppet spring 160. Hence, the hydraulic fluid of the space 120a does not flow into the second orifice 131, so that the inlet side pressure is maintained to be equal to the chamber side pressure.
Here, since the pressure receiving area of the main poppet 120 on which the inlet side pressure is exerted is smaller than that of the main poppet 120 on which the chamber side pressure is exerted, the main poppet 120 is supported toward the right side in the sleeve 110 by the main poppet spring 122. Thus, the tank fluid channel 112 is kept closed by the main poppet 120.
Meanwhile, when the working tool (not shown) of the heavy equipment reaches the maximum stroke, system pressure of the heavy equipment is increased. Thus, the pressure on the side of the high-pressure inlet 111 is increased, so that both the inlet side pressure and the chamber side pressure reaches a pressure higher than the setting pressure set by the pilot poppet spring 160. At this time, the hydraulic fluid reacts against the spring force of the pilot poppet spring 160 to move the pilot poppet 150 to the left side, so that the seat fluid channel 142 of the seat 140 is opened.
Therefore, since the hydraulic fluid is returned to the tank 201 via the seat fluid channel 142 and the tank inlet 181, the hydraulic fluid introduced into the space 120a through the first orifice 121 is subjected to resistance. As a result, the chamber side pressure becomes lower than the inlet side pressure. If so, the force applied to the pressure receiving surface of the main poppet 120 on which the inlet side pressure is exerted exceeds the force applied to the pressure receiving surface of the main poppet 120 on which the chamber side pressure is exerted. Accordingly, the main poppet 120 moves to the left side, and the hydraulic fluid is returned through the tank fluid channel 112 to the tank 201.
The viscosity of hydraulic fluid used in the hydraulic system is changed according to a temperature. Specifically, when the equipment is used in winter, the viscosity of the hydraulic fluid is increased as the temperature is lowered. On the other hand, when the equipment is used in summer, the viscosity of the hydraulic fluid is lowered as the temperature is increased. Therefore, as the viscosity of the hydraulic fluid is greatly changed due to the temperature change, the variation of a flow rate of the hydraulic fluid passing through the fluid channel is increased.
It is well known that failures occurring at hydraulic equipment are, for the most part, due to the contamination of the oil used as the hydraulic fluid. The hydraulic fluid circulating the interior of the hydraulic system contains foreign materials such as dust, air, etc. For instance, fine particulates such as dusts block drain openings or fluid channels, thus resulting from malfunction. In addition, the foreign materials such as molding sands are introduced into a pressure chamber of the relief valve, thus obstructing the operation of the main poppet or causing damage of the seat.
According to the conventional relief valve 100, the fluid passage, through which the hydraulic fluid of the high-pressure inlet 111 is introduced into the space 120a in the rear of the main poppet 120, is formed by the first orifice 121 provided on the front end of the main poppet 120. For this reason, the foreign materials contained in the hydraulic fluid are introduced into the interior of the relief valve 100 through the first orifice 121, so that operation of the relief valve is deteriorated.
In order to solve the problem caused by the foreign materials contained in the hydraulic fluid, a tank filter is fitted on the entry portion of the hydraulic pump, or an oil filter functioning as a line filter is fitted in the pipes of the hydraulic pump. Nevertheless, many foreign materials are still contained in the hydraulic fluid. Furthermore, the oil filter used in the hydraulic system is expensive, which acts as a cause of increasing production costs as well as the number of parts.